Gas flares are commonly located at production facilities, refineries, processing plants, and the like for disposing of combustible waste gases and other combustible gas streams that are diverted due to venting requirements, shut-downs, upsets, and/or emergencies. Such flares are often operated in a smokeless or near smokeless manner, which can be largely achieved by making sure that the flammable gas to be discharged and burned (“flare gas”) is admixed with enough air to sufficiently oxidize the gas. Pressure assisted multipoint ground flares (MPGF) have long been used in the petrochemical industry, as well as gas plants and refineries for the safe disposal of vent gases during upset conditions. A properly designed MPGF can achieve 100% smokeless operation under all flow conditions for which it has been designed. This type of flare has low public profile compared to a typical elevated flare because these systems may have no visible flame outside the plant. Additionally, the higher destruction efficiency of an MPGF can significantly reduce continuous plant emissions. These flare systems are comprised of a radiation fence—also known as a wind fence—a distribution manifold with fail-open valves, multiple smaller manifolds (or runners), which terminate in flare burners, and a control system that operates the staging valves based on the supply pressure. Surrounding the field is the radiation fence. In the foreground, the elevated and shielded horizontal pipes (just above grade) are the runners and the burners are mounted on the smaller vertical pipes.
A typical flare apparatus includes one or more flare burners and a pilot. As gases exit the flare burners, the gases mix with the oxygen and combust (via the flame from the pilot). Some flare burners use various methods in an attempt to provide sufficient oxygen in a combustion zone of a flare burner to help minimize the formation of smoke.
For example, in some flare burners, the size of the flare burner is larger. However, as a result of the large size of the flare burner, a significant amount of ground space is often required for the flare burner. This problem is increased when multiple flare burners are used, with the burner array requiring a large area of ground space.
In some flare burners, the flame that is produced is very high. Not only is the high flame height undesirable, but the high flame height requires a higher fence around the flare burner area. The higher fence is more expensive. The higher flow of waste gas in the center of the flare tip can also increase the oxygen requirements at the center of the flare tip. This can increase the propensity of the flare to smoke.
Furthermore, many large flare burner areas require a large amount of piping and multiple valves. The required piping and valves increase the capital cost associated with the flare burner. Additionally, these types of flare burners also may require welded joints and attachment points. This results in a flare burner that is complex to assemble and costs more.
In addition, many flare burners are noisy mainly due to both jet noise and combustion noise. While the jet noise (the noise associated with the speed of the gases exiting the burner) may not be able to be lowered, it is believed that the combustion noise (associated with the mixing of the air and fuel gases) can be lowered and still provide an acceptable flame.
In addition, it has been found that flares that are located in close proximity to ancillary equipment may cast substantial amounts of thermal radiation on the equipment during normal operation. If a flame center of radiation is moved closer to such equipment as fences, the radiation will increase and equipment may be damaged. This invention solves the problem by utilizing an asymmetrical disposition of fuel gas combined with an offset riser such that a substantial portion of the radiation is cast away from the ancillary equipment to grade.
Some fuel gas is still in vertical alignment with the distribution manifold in order to facilitate cross-lighting of individual flare burners.
An additional issue is that frequently multiple burners are used. When multiple burners are used in conjunction the momentum of the flames flowing in unison tends to merge them together and increase their length due to a lack of access to air. In the case of a multi-point flare this can mean that burner flames that are in isolation would have a length less than that of the surrounding radiation fence can merge and have a resultant length that is taller than that of the radiation fence.
Pressure-assisted flare burners rely on high velocity vent gas jets for the entrainment of combustion air to provide smokeless operation. The minimum pressure, and therefore minimum port exit velocity, at which the burner can operate without producing smoke from the flame is a critical design feature.
A lower smokeless operating pressure for a flare burner results in a wider operating range for a given stage, so, therefore, can reduce the number of stages required for a properly functioning smokeless flare field. There is an additional benefit of reduced heat load on the flare components due to adequate vent gas and air mixing even at low flows. The de-staging pressure is the minimum operating pressure of the flare system for which smokeless performance should be expected. The maximum operating pressure that produced smoke for any vent gas tested was 42% of this de-staging pressure. Accounting for the variability among the tested vent gases, there is a 0.01% (122.87 ppm) chance of visible smoke across vent gas types during a de-staging event. The probability of visible smoke from the flare burner is less than the statistical analysis suggests, given that the most common type of vent gas that produces the most smoke is contained within the data set at 42% of the de-stage pressure. Although visible smoke could occur near the burner, it still may not rise above the flare fence before dissipating. The Galaxy burner produces essentially no visible smoke for any of the vent gas compositions tested.
Therefore, it would be desirable to have a flare burner for combustible gas that addresses each of these issues.